The present invention relates to carrier particles for use in electrostatic image development, and an electrostatic image developer including the same. The carrier particles are included, together with toner particles, in a two-component system developer which is a kind of dry type developer used in an electrophotographic copying machine and the like.
A two-component system developer used in electrophotography includes toner particles and carrier particles. The toner particles are in a form of powder which make an electrostatic latent image on a photoreceptor visible. The toner particles include a binder resin such as a styrene-acrylic copolymer and a polyester-type resin, and pigment such as carbon black and the like as an essential component. Carrier particles play several roles such as to convey toner particles to a developing area, to produce friction with toner particles so that toner particles are charged, to be used as electrodes in the development of an electrostatic latent image, and to remove unnecessary toner particles on the photoreceptor. Examples of carrier particles include magnetic powder such as iron powder, ferrite and the like. Recently, using such magnetic powder as a core, the magnetic powder is coated with a resin-type coating material, and thus-obtained carrier particles have coming into wide use.
Conventionally, a coating material including a silicone resin, a fluoro resin, and the like as an essential component has been mainly used due to their low adhesiveness to a binder resin (for example, a polystyrene resin, polyester, or a styrene-acrylic resin, and the like) contained in toner particles. These resins have been used for the purpose of avoiding the attachment between toner particles and carrier particles. If the toner particles attach onto the carrier particles, there are problems of the spent toner, and the contamination of the carrier particles by the toner particles so that the toner particles are not sufficiently charged by friction.
When the coating layer is made of a coating material containing these resins having low adhesivness, however, the coating layer has only low adhesiveness to the core of the carrier particle. With weak adhesivenss, a coating layer is likely to peel off the core, so that the electric resistance of the carrier particle is decreased and the charge level of the toner particle changes with time. In order to solve such problems, there have been suggestions to use a coating material including a silane coupling agent alone or in a mixture with a silicone resin. For example, Japanese Patent Publication No. 7-19079 suggests the use of a coating material including silane coupling agent alone. Japanese Laid-Open Patent Publiation No. 7-160058 suggests the use of a coating material including a silane coupling agent and alkoxysiloxane. Japanese Laid-Open Patent Publication No. 7-104522 suggests the use of a coating material including a silane coupling agent and a silicone resin.
The organic coating layer made of such coating materials has relatively enhanced adhesiveness to the inorganic core; however, is poor in abrasion resistance and impact resistance, so that during repeated use, the coating layer peel off the core. The carrier particle with such a coating layer is poor in durability. There is also a problem remaining unsolved that the toner particles are not sufficiently charged by friction at high temperature under high humidity, so that the obtained image has fog.
The prevent invention has been conducted to solve the above-described problems, and the objective thereof is to provide carrier particles excellent in durability, with which a clear image with no fog can be formed for a long period of time even at high temperature under high humidity as well as at normal temperature under normal humidity.